The present invention relates to a semiconductor device incorporating a contact and to a method of fabricating such a semiconductor device.
In CMOS semiconductor devices and processes it is known to employ electrical contacts between polysilicon, such as a polysilicon signal line, which signal line may integrally connect with a polysilicon gate layer, and diffusion silicon, such as a doped region of the silicon substrate of the device, which doped region is an active area of the device. Such polysilicon/diffusion silicon contacts have in the past been produced by forming two contact holes in the dielectric layer which overlies the polysilicon and the diffusion silicon, one contact hole extending down to the upper surface of the polysilicon and another contact hole spaced from the first contact hole extending down to the diffusion silicon. A metal wire is then located in the two contact holes and extends therebetween over the region of the dielectric layer which is located between the two contact holes. The known arrangement has layout disadvantages because each contact is subject to spacing constraints which are imposed by the requirement electrically to isolate any two contacts.
It is also known to employ a local interconnect layer, for example of titanium nitride or titanium silicide, as a strap which connects the polysilicon to the diffusion silicon with the strap extending over the upper surface of the polysilicon and of the diffusion silicon. The local interconnect layer is formed after the patterning of the polysilicon but before the deposition of the dielectric layer whereby the strap is covered by the dielectric layer. This arrangement suffers from the disadvantage that the manufacturing process is relatively complex since additional deposition and etch steps for the local interconnect layer are required.
In addition, it is also known to employ local removal of gate oxide between polysilicon and active silicon to produce a direct electrically conducting contact which is a buried contact. However, this method is limited to connecting N+ polysilicon to N+ active silicon because, if opposite types are attempted to be connected, subsequent heat treatment steps at greater than 800.degree. C. can cause parasitic diodes to be formed in the diffusion silicon.
It is well known to employ an aluminium metal contact which is deposited by sputtering in a contact hole in a dielectric layer and extends down either to the polysilicon i.e. to a signal line or to the diffusion silicon substrate i.e. to an active area. It has been proposed to provide a contact between the polysilicon and the diffusion silicon by forming a contact of aluminium metal in the form of a strap which extends over both the polysilicon and the diffusion silicon. However, such aluminium contacts suffer from the disadvantage that the lateral dimensions of the contact hole and the contact are relatively large in order to enable the aluminium reliably to form the contact with effective electrical connections.